Conventional semiconductor doping is not a deterministic process, i.e. it creates a statistical distribution of dopants. In the case of field-effect devices, stochastic doping enables reliable device performance by ensuring a reproducible distribution of large numbers of dopant atoms in the channel region. However, conventional doping processes, such as traditional diffusion or implantation techniques, cannot be used to control the atomic scale positioning of dopant atoms. In conventional FET devices, the exact number of doping atoms in a given region is determined by chance, constrained by the requirements that the average doping over a large number of small regions must be the correct macroscopic average. As feature sizes are scaled to a few nanometers (nm), the traditional stochastic approach to doping is presenting problems.
Some of the reasons that conventional doping technology imposes limits on scaled sub-100 nm FET integrated circuits are related to the number and position of individual dopant atoms in the channel. First, consider the impact of dopant numbers in ultra-small FETs. At these small dimensions, reliable FETs require large channel doping densities to prevent the punch-through effect. However, large channel doping densities in such conventionally scaled FETs adversely decrease device performance, for example, due to multiple scattering that decreases channel mobility. Hence, conventional stochastic doping technology limits the feasibility of achieving concurrent improvements in the performance of scaled, ultra-small FETs.
Super steep retrograde doping technology offers some advantages over conventionally scaled devices. The channels of sub-70 nm FETs with super steep retrograde doping consist of a very thin surface layer, with a lower dopant concentration of 1017 cm−3, that changes abruptly to an underlying heavily doped layer, with a dopant concentration of about 1019 cm−3. The thickness of the lightly doped surface layer should be less than the source-drain extension depth. Based on the data in Table 1, derived from the 2001 International Technology Roadmap for Semiconductors (ITRS), the thickness of the retrograde doping layer is less than 10 nm and 5 nm at the 80 nm and 32 nm technology nodes, respectively. The purpose of the retrograde layer is to create a thin, lightly doped region of high mobility on top of a heavily doped, lower-mobility layer. In conventional, stochastically doped devices, carrier scattering increases and mobility decreases with the number of dopant atoms in the channel. Conventional super steep retrograde doped devices seek to maintain a dopant concentration of ˜1017 dopant atoms per square centimeter in this retrograde layer. In conventional stochastically doped devices of this size, this doping concentration exhibits acceptable carrier mobility. ITRS specifications for sub-70 nm FET channel retrograde doping profiles
Drain exten-sion lateralRetrogradeNumber ofITRS Technol-Gate lengthDrain exten-abruptnesschannelatoms in retro-ogy Node(nm)sion (nm)(nm/decade)depth (nm)grade channel80 nm4513-233.513-191065 nm3510-172.810-15432 nm185-91.45-71
Next, consider the impact of dopant number and position on the variability of small device performance characteristics. According to Table 1, as devices scale below the 80 nm ITRS node, the number of dopant atoms in the retrograde channel is less than ten. Such a small number of dopants in the thin surface layer cannot be obtained reproducibly using conventional doping techniques. The device parameters are therefore sensitive to statistical fluctuations (proportional to the square root of the number of dopants). At the 100 nanometer ITRS node, the operating regime is such that discrete dopant fluctuations lead to substantial variations in threshold voltage of about 20-50 mV. Below the 80 nm node the threshold voltage variation can be 25-100% of the operating voltage. Taking into account the uncertainty in dopant position, in addition to the uncertainty in the number of dopants, will make the parameter variability even worse. Uncertainty of the individual transistor's parameters in one chip imposes a practical limit to scaling, due to the difficulty of design, fabrication and operation of complex systems based on non-identical FETs. In complex ICs, the probability that all transistors are identical and that a circuit will function properly, within given specifications, sharply decreases with decreasing channel length. Therefore, new ways of doping must be developed if practical ultra-small field-effect devices are to be realized.